In order to reduce volume and to unwind or rewind the fishing line as quickly as possible, fishing reels currently available are all designed to unwind or rewind the fishing line by manually operating a crank handle to drive a small-module single-gear driving mechanism which in turn drives a rotor to rotate. Meanwhile, for the purpose to reduce the weight, the gear is made to have a limited strength. This kind of structure works well for fishing of small size fish. However, when fishing a fish of larger size, the user tends to subconsciously apply more force to the crank handle. In this case, top portions of the pinion teeth are subjected to a substantial force that may possibly cause significant wear or even fracture of the top portions, which is undesirable for the user.